If anyone interested I tracked down the proper setting.
Turn on radio
Press Select for 8 seconds until radio enters service mode
Turn the control knob until option GAL is selected.
Options are 1 - 6 using preset buttons. Default on mine was 3. I prefer slightly lower so selected 2
Once set turn off radio to leave service mode.
Probably best not to set other options. I guess GAL is lost in translation from German
Found the info on the BSW website. It was for the BM53 radio module but worked just as well on my BM54

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You can set the volume for Traffic Announcement as well in the same menu.